use a helper script as recommended on https://docs.docker.com/articles/dockerfile_best-practices/#entrypoint
diff --git a/jenkins.sh b/jenkins.sh
new file mode 100755
index 0000000..791f938
--- /dev/null
+++ b/jenkins.sh
@@ -0,0 +1,4 @@
+#! /bin/bash
+
+exec java $JAVA_OPTS -jar /usr/share/jenkins/jenkins.war --extractedFilesFolder=/var/lib/jenkins --prefix=$JENKINS_PREFIX $@
+